(a) Field of the Invention
The present invention relates to a UV nanoimprint lithography process, and more particularly, to a UV nanoimprint lithography process in which nanostructures are produced by pressing an elementwise embossed stamp on a resist deposited on a substrate to transfer nanostructures.
(b) Description of the Related Art
UV nanoimprint lithography technology enables the economic and effective production of nanostructures. To perform UV nanoimprint lithography, it is necessary to use nanoscale materials technology, stamp manufacturing technology, anti-adhesive layer technology, etching technology, measurement analysis technology, etc. It is also necessary to use nanoscale precision control technology in the process.
Nanoimprint lithography has a high possibility of being applied to the production of high-speed nanoscale MOSFETs (metal-oxide-semiconductor field-effect transistors), MESFETs (metal-semiconductor field-effect transistors), high density magneto-registers, high density CDs (compact disks), nanoscale MSM PDs (metal-semiconductor-metal photodetectors), high speed single-electron transistor memories, etc.
In the nanoimprint process first developed in 1996 by Chou, et al. of Princeton University, a stamp, which has a nanoscale structure manufactured by using the electron beam lithography process, is pressed onto a substrate, which is coated with a thin layer of PMMA (polymethylmethacrylate) in a high temperature environment. After being cooled, the stamp is separated from the resist. Accordingly, the nanostructures on the stamp are transferred onto the resist. Using an anisotropic etching process, they are then transferred onto the substrate, which is generally a silicon wafer.
In 2001, Chou et al. developed the laser-assisted direct imprint (LADI), a nanoimprint technique. This technique uses a single 20 ns excimer laser of a 308 nm wavelength to instantly melt a silicon wafer or a resist coated on a silicon wafer to perform imprinting with a transparent stamp. Further, in a similar process of the nanosecond laser-assisted nanoimprint lithography (LA-NIL) applied to polymers, a nanostructure of 100 nm in width and 90 nm in depth is imprinted on a resist of a polymer.
These nanoimprint technologies are performed at high temperatures. In the development of semiconductor devices requiring multi-layer operations, thermal deformation caused by the high temperatures makes it difficult to successfully perform the multi-layer alignment. Further, in order to perform imprinting of a resist with a high viscosity, a high pressure approximately as high as 30 bar is needed, which is liable to damage to the previously produced nanostructure. An opaque stamp used in these processes makes the multilayer alignment even harder.
To address these problems, Sreenivasan et al. of the University of Texas at Austin developed the step and flash imprint lithography (SFIL) in 1999. In SFIL, UV-curable resins are used to produce nanostructures at room temperature and at low pressure. Transparent materials transmitting UV lights such as quartz and Pyrex glass, etc. are used as the stamp material.
In SFIL, a transfer layer is first spin-coated on a silicon substrate. Next, in a state where a transparent stamp is maintained at a predetermined small gap with the transfer layer, a UV-curable resin with a low viscosity is filled in nanostructures of the stamp and by capillary force. When filling of the nanostructure is complete, the stamp is contacted to the transfer layer and ultraviolet rays are irradiated onto the stamp to harden the resin. The stamp is then separated from the transfer layer, followed by an etching process and a lift-off process to thereby complete patterning of the substrate.
SFIL is a step-and-repeat type nanoimprint process, in which a stamp, relatively smaller than the substrate, is used to repeatedly perform imprinting over the entire substrate. Although nanostructures of the stamp are quickly filled due to the small area of the stamp, the need to repeatedly align the stamp and perform multiple imprinting processes for a substrate increases the overall production time.
In order to effectively perform imprinting on a large substrate, with reference to FIGS. 19A and 19B, nanostructures 103 should be formed on a single stamp 6 as large as the substrate, and the stamp 6 be pressed against a resist 20 deposited on an upper surface of the substrate 5. Nanostructures corresponding to the shape of the nanostructures 103 formed on the stamp 6 are therefore transferred onto the substrate 5. However, the resist 20, which has a low viscosity, flows only toward edges of the substrate 5 by the pressure applied by the stamp 6 as shown in FIG. 19B (in the direction of the arrows). Thus, in case that the distribution of the resist 20 in inner areas of the substrate 5 is uneven, or there are impurities such as air in the resist 20, the resist 20 cannot be fully filled in the nanostructures 103 formed on the stamp 6.
Because of flatness errors of a stamp for UV nanoimprint lithography and the working surface of a substrate (e.g., 20-30 μm for a Si wafer substrate), the resist cannot be uniformly imprinted by the stamp during the imprinting process. In order to prevent such non-uniform nanoimprinting of the resist by the small stamp in SFIL, the distances between the stamp face and the substrate is controlled using four distance sensors, each of which is mounted on each side of the stamp. The positioning of the stamp is then varied according to the resulting distance measurements to thereby maximally level the stamp face with respect to the substrate surface. That is, imprinting is performed after adjusting the planar angles of the stamp surface on which nanostructures are engraved according to the waviness of the substrate surface.
However, increase in sizes of the stamp and the substrate results in greater flatness errors such that the resist has even more areas of insufficient and non-uniform imprinting, i.e., the resist does not fully or not uniformly fill the nanostructures. Also, with the non-uniformly imprinted resist on a substrate, difficulties arise in the etching process, which is used for transcribing the nanostructures on the substrate.